There is a need for a tape cartridge which houses a pair of tape reel hubs and a tape wound convolutely upon those hubs and extending between them together with an arrangement by which the hubs can be rotated to transport the tape between the reels past magnetic recording and readout heads. Tape cassettes were developed to fill that need. However, to construct a tape cassette suitable for use where very high fidelity is required has proven to be difficult. Thus, for example, it has been difficult to produce suitable tape cassettes for use in recording and playing back large quantities of digital data at reasonably high transport speeds. It has been difficult to produce a tape cassette in which the tape transport speed is maintained highly uniform and in which tension is maintained at the same level during starting and stopping and reversing operations at which it is maintained during tape transport.
To meet these requirements, the technique of driving the peripheral surfaces of the supply and take-up rolls was developed. To do that, while maintaining a uniform tension in the portion of the tape being transported from one roll to the other, requires that the take-up roll tend to be driven at a slightly faster speed than the speed at which the supply reel is driven. That can be accomplished with a drive belt which extends between a drive roller and an idler roller, and which is arranged so that a portion of the belt extends over the peripheral surface of each of the tape rolls. The belt must be coupled with some means for altering the belt speed at the two tape rolls, or some means must be provided for altering the speed at which the belt drives the rolls, so that the take-up roll tends to rotate faster than does the supply roll, whereby tension is maintained in the section of tape that is transported between them.
That differential in tape roll velocity is accomplished in Von Behran, U.S. Pat. No. 3,692,255 by frictionally opposing the idler roller. In Grant U.S. Pat. No. 3,808,902 it is accomplished with a belt having appreciable thickness and utilizing the difference in velocity of the inner and outer surfaces of the belt as it is transported from the drive roller to the tape rolls.
It is difficult in a system that employs a friction roller to develop a differential tape roll speed to maintain the friction uniform under changing environmental conditions, and it is difficult in such a system to prevent undamped oscillation of the drive belt, and therefore of the velocity at which tape is transported. Such systems suffer tape transport velocity change and the transport speed, or the data density on the tape, must be reduced. On the other hand, the general physical arrangement of the cartridge case, or shell, and the arrangement of the belt drive roller at one edge of the case that is depicted in the Von Behran patent provides a number of advantages, and that physical arrangement has enjoyed wide acceptance.
The Grant Radial Differential Tape Drive, described in U.S. Pat. No. 3,808,902, is capable of a superior performance, but the patent does not deal with the question of packaging that tape drive system in a cartridge case. To combine the case of Von Behran patent, or a case with similar advantages, with a drive system such as that shown in the Grant patent would be ideal.